The broad objective of our work is to understand the neural regulation of the heart under a variety of natural and experimental conditions. The specific objective of the present proposal is to elucidate the phase dependent effects of efferent vagal activity on various aspects of cardiac function. We shall determine the influence of the basal heart rate on the phase-dependency; specifically, we shall determine whether periodic bursts of vagal activity will actually accelerate the heart above the basal rate, provided that the basal rate is sufficiently slow and that the activity bursts are timed properly within the cardiac cycle. We shall define precisely, within the cycle of S-A nodal pacemaker activity, the time of greatest refractoriness to neurally released acetylcholine. We shall also determine the influence of various levels of tonic cardiac sympathetic activity on the phase-dependent response to periodic bursts of vagal activity. We shall also assess the bilateral symmetry of the phase-dependent responses to vagal stimulation, and the bilateral symmetry of the cardiac vagal-sympathetic interactions. The phase-dependent reflex responses to aortic nerve stimulation will be compared with those to carotid sinus nerve stimulation. Finally, the phase-dependent effects of efferent vagal stimulation on atrial contractility will be determined.